Rain water harvesting and grey water re-use are widely known applications both for water supply and water end use optimisation. In this study, possibility of water savings evaluated in case of grey water re-use and rain water harvesting taken as an integrated system. Precipitation values and domestic water end uses are required to calculate total water savings of any arrangement in water use model. An estimated distribution of domestic water end use estimated conducting a literature survey on related studies. Potential rain water harvesting volume for Ankara city was calculated using available precipitation data. This water volume was accepted as a secondary water resource for drinking water utility. Rain water harvesting and grey water re-use requires storage. Required reservoir volume to store rain water was calculated under two different scenarios. In the first scenario, washing machine and dishes washer was fed by rainwater, and in the second one an extra volume for irrigation was reserved. Reservoir volumes to store effluents of washing machine, dishes washer, bathing, shower and faucets were calculated under two different scenarios. In this way, water use scenarios were shaped based on four different options. It is estimated that rain water harvesting method is capable to feed only 70% of total domestic consumption in Ankara. Thus, system should be designed on low demanding side. In this case, it is not feasible to select corresponding reservoir volume. However, water supply still will be assured 90% even if the reservoir volumes selected in a half. It is estimated that rain water harvesting and grey water reuse altogether offer a chance of water savings between 40% and 46%.